Small plops, big splashes

A ball dropped into water can make a tiny plop or a huge splash, depending on what it's coated with.

Physicists in France watched and listened as two small glass balls a few centimetres in diameter were dropped into water from
a height of 1.25 metres. One ball was coated in a thin layer of water-repellent silane. The other was very smooth and clean,
so that the sphere became completely wet when it hit the water.

The water-repelling sphere made a huge air pocket behind it as it hit the water, and a loud splash. The wet sphere didn't
create the air pocket and only made a quiet plop.

This result surprised team member Lydéric Bocquet at the University of Lyon. Traditional hydrodynamics generally ignores surface
effects, he says, because the behaviour depends on the speed at which the ball drops.

In fact, as he and his colleagues report in Nature Physics1, the reverse turns out to be true. Above a certain speed, the size of the splash depends on the chemical nature of the surface.

Listening to molecules

As the ball enters the water, a thin film of water climbs up the side of the ball. But this film behaves very differently
on different surfaces.

"Water prefers to stay together than go over the surface of the water-repellent ball," says Bocquet, "It's pushed away from
the ball." As this happens, an air pocket is formed in the ball's wake.

With the large air pocket comes a large splash. For the perfectly wetting ball, however, the film sticks close to the surface,
and no cavity forms, resulting in a quiet plop.

Bocquet tested spheres with coatings ranging from water-attracting to water-repelling, measuring the splashes with a microphone.
As the balls became more water repellent, their splashes became louder.

"In some sense you can hear the molecular details," Bocquet says.

Below a certain velocity, the air pocket doesn't form. This threshold speed depends on the angle of the water film as it climbs
up the ball, and so on the water-repellent nature of the ball.

But with the most hydrophobic coating, the speed at which the ball hits the water makes no difference. The splash is always
there.

These experiments could prove useful for the military, Bocquet predicts. Torpedoes, for example, need to have a very clean,
easily wetted coating at the tip, because air bubbles here can prevent the torpedo finding its target.